Warehouse racks are much like bookshelves in that they have a fixed outer shell with adjustable-height shelves. As with bookshelves, horizontal members (the rack levels, such as shelves) on a warehouse rack can be placed at arbitrary heights. Warehouse racks have been used with standard shelf heights to define rack openings that are uniform in size throughout a rack. For example, warehouse racks have traditionally been configured so that every rack opening has the same size.
Warehouse racks are used to store pallets of goods. Pallets are generally flat transport structures that support goods in a stable manner and that also include structures that are adapted to fit forklifts and/or other devices/machines to move pallets. Stacked on top of pallets are packages of various products. The weight and volume stacked on a pallet varies, resulting in a mix of heights for a given inventory profile. Warehouse racks have been configured so that the openings have a standard width and depth that will accommodate pallets being picked and put by forklifts or other machines/devices. The height of rack openings have traditionally been standardized as well, so that each opening is uniform and interchangeable.